Download medLecture No 4

NAJRAN UNIVERSITY
College of Medicine
Microbiology &Immunology Course
Lecture No. 4
By
Dr. Ahmed Morad Asaad
Professor of Microbiology
Bacterial physiology and metabolism
Bacteria have certain requirements In order to grow
and multiply. These requirements include:
1- Nutrients
2- O2 requirement
3- CO2 requirement
4- Suitable Temperature
5- Suitable pH
Nutrients
Autotrophic Bacteria
Heterotrophic Bacteria
According
to nutrients,
bacteria
arerequire
classified
into:
- They
can utilize
simple
- They
complex
organic
inorganic materials (CO2 as the
source of carbon, and
ammonium salts as the source of
nitrogen).
- Can form complex organic
metabolites from these simple
materials.
- Autotrophic bacteria may
derive their energy from light
(e.g. photosynthetic bacteria) or
from chemical reactions by
oxidation of inorganic materials
(e.g. chemolithotrophic
bacteria).
materials derived from animal
and plant sources.
Cannot synthesize
complex organic substances from
the simple inorganic materials.
They derive their energy
by the oxidation or fermentation
of organic compounds (e.g.
glucose).
- Most bacteria of medical
importance are "heterotrophic
bacteria".
O2 requirement
According to O2 requirement, bacteria are classified
into 4 types:
Type
1. Obligate aerobes
Definition
Bacteria that grow &
multiply only in
presence of free O2
(i.e. O2 is essential)
2.Obligate anaerobes Bacteria that can not
grow in presence of
O2
(i.e.
require
complete absence of
O2)
Examples
Mycobacterium
tuberculosis & Vibrio
chlorae
Clostridia spp.
Type
Definition
Examples
3.
Facultative Bacteria which can Most bacteria of
anaerobes
utilize
O2
when medical Importance
present, but can also
grow & multiply in
its absence
4- Microaerophilic
Bacteria
which Campylobacter
require
low
O2
tension (lower than
that present in the
atmosphere)
What is the difference between obligate aerobic and
obligate anaerobic bacteria?
In aerobic bacteria, O2 is utilized as the final oxidizing agent.
2 H2O + 2 O2 → 2 H2O2 + O-2 (Toxic substances)
Aerobes and facultative anaerobes contain certain enzymes, e.g.
superoxide dismutase, catalase and peroxidase, which protect
them from the effect of these toxic materials.
Anaerobic organisms do not possess these protective enzymes and
the presence of O2 will be harmful to them due to the production
of those toxic materials.
Thus in anaerobic bacteria, the oxidizing agent is not O2 but other
inorganic or organic compounds.
CO2 requirement
- Most bacteria require CO2 in just as a small concentration
as that present in air.
- Carboxyphilic bacteria need high CO2 concentrations., e.g.
Neisseria: requires 5% CO2.
Brucella abortus: requires 20% CO2.
Temperature requirement
According to temperature, bacteria are classified into:
Bacteria
Mesophilic bacteria
Psychrophilic bacteria
Thermophilic bacteria
Temperature
range
18-42ºC
5-30ºC
25-80ºC
Optimum
temperature
37ºC
15-20ºC
50-60ºC
What is the type of bacteria of medical importance in
relation to temperature?
pH requirement
-Most bacteria of medical importance can grow and
multiply within a pH range of 7.2 - 7.6.
-Some bacterial species require:
Acidic pH for their maximal growth (e.g. lactobacilli).
Alkaline pH (e.g. Vibrios).
Bacterial growth curve
When bacteria are transferred to a suitable fluid medium
and the number of viable bacterial/ml is calculated at
regular time intervals, a growth curve will be obtained
which can be divided into 4 phases:
1- The lag phase :
- During this phase, there is no or little bacterial
multiplication.
- It is the period between inoculation of the medium
with the organism and beginning of active
multiplication.
- In this phase the organism adapts itself to the new
environment by synthesizing new enzymes and
macromolecules required for metabolism.
2- The logarithmic (exponential) phase :
- It is the most active phase of bacterial growth.
- The number of bacteria increases steadily by time.
- This phase continues until :
The available nutrients are exhausted, and/or
The toxic waste products are accumulated.
3- The stationary phase :
-During this phase the rate of bacterial growth is slow
due to exhaustion of nutrients and accumulation of
waste metabolites.
-The number of dying bacteria = the number of newly
formed bacteria.
4- The phase of decline :
-When exhaustion of nutrients and accumulation of
waste products continue, the number of dying bacteria
exceeds the number of newly formed bacteria.
Clinical significance of bacterial growth curve:
Phases of growth curve
(in vitro)
The lag phase of the curve
The logarithmic &
stationary phase
The phase of decline
Stages of diseases &
infections (in vivo)
The incubation period of
the disease
The clinical signs &
symptoms
The recovery &
convalescence stage